A conventional image forming device, for instance a digital copying machine, comprises an image reading unit, a storage device for temporarily storing the image data read by the image reading unit, and an output means for reading and delivering onto a transfer medium the image data stored in the storage device. A nonvolatile memory, such as a hard disk, is generally used as the storage device.
In such a copier, the image data remains in the storage device after the image data output until it is overwritten by the image data of the next document. If, for instance, the copier is disposed of in this state, and the internal hard disk or the like is taken out, in some cases, the image data remaining on the disc is analyzed and externally disclosed. This problem is serious, in particular when image data containing highly confidential information has been handled.
Thus, conventionally, a method has already been proposed in which a confidential document mode is provided, and if this mode has been set, the image data is deleted when the image data output is finished, and a method has been proposed in which the image data is deleted while the image forming device is idle.
However, erasing image data requires some time. In particular, if a hard disk is used as the storage device, for deleting image data that has been stored, an A4 size image data requires on the order of 0.75 seconds to be deleted, and the time required to delete 80 pages of image data is on the order of 60 seconds.
In addition, while the image data is being deleted, the next process such as printing cannot be carried out continuously, leading to the problem of drop in processing efficiency. An object of the present invention is to prevent external disclosure of image data while avoiding a drop in processing efficiency in an image forming device wherein image data is stored in a storage device during output.